Just wanted to thank Samu-rai for his excellent write up - after months of meaning to, I eventually got round to doing this (as the kids are really starting to get too inquisitive now!) and although I'm using MQ5, this was of great help, as the process is very similar.
Thought I would share what I've done a little, as it is slightly different - I moved out the movies (& TV shows) that I was happy for the kids to browse and watch. This took far less time than I thought it would, partly because; as I wasn't moving them to a different drive, just a different folder on the same drive, there was no time taken up moving them. It also didn't take long going through the movies and deciding which ones the kids could watch - I have always had my movies sorted into different folders for genre, so it was quick to just pick the whole of Animation and the whole of Family and then go through the odd ones in Action (Indiana Jones etc..), Fantasy (Harry Potter etc..) and Science-Fiction (Star Wars etc..) that I was happy for them to watch.
With that done and adding the new sources to XBMC I then had to tackle these smart playlists, which were all new to me - personally, I couldn't get on with the suggested path contains and then browse for the source (they just wouldn't show up!), so instead, I did path contains and simply wrote "Kids". For each of the new folders that the kids movies were put in earlier, I had named them Kids Blu-Ray, Kids DVDs and Kids HDTV etc... so this worked like a treat and this smart playlist was added to the main menu.
The next issue was that of genre - I've always tended to initially search for movies via genre (sometimes year for myself) and the kids seem to have copied this approach. For Animation and Family films it was easy as I could simply create a smart playlist where genre is Animation etc... and then add this video playlist as a sub menu entry under the Kids Movies main menu item.
For the other genres, where I didn't want every movie to be shown from that genre, I created a smart playlist for each with the genre rule and an additional rule with path contains "Kids" and then added them to the submenu;
I could've used the tags feature, but this would have taken longer, although I might use it for say Pixar/DreamWorks movies etc... in the future.
So, the kids now have 6 genres to browse when finding a movie, they think they've got more movies than before and we're not worrying they might come across a Halloween, Hellraiser etc...!!
Rather than creating playlists for our movies (& TV shows), I left the existing menu items in place, so if we go in through them they show all movies including the kids ones. And we have decided to simply turn off these menu items via the customize main menu option each day so they are effectively only showing after say 7pm each evening - so the kids can't look through them during the day, which is keeping the wife happy!
I also decided to turn off the recently added widget for these video playlists - I was really happy to see that Marcos had made it possible to turn them off for the playlists but they remain working for the "normal" movies menu item - it keeps it cleaner and less confusing for the kids in my opinion, but we can still have the feature on our menu item.
I had previously put in a feature request for password protection on movies over a certain rating and although there was some interest, nothing seems to have happened as yet. I still think this would be a great addition to XBMC in the future, but in the meantime this workaround as explained by Samu-rai works great.
Many thanks again to Samu-rai and also to Marcos for his great skins.